This invention relates to a multiplexed adaptive array and array processor for use in high frequency communications systems and in particular to a coherent dual automatic gain control circuit that maintains stable adaptive loop gain at all signal input levels.
Communication channels in the HF band are susceptible to various types of interference. This interference can either be deliberate jamming or another undesired signal transmission. The applicability of adaptive processing techniques to HF sidelobe cancellation as a means for obviating the effect of such interference has been investigated experimentally. It has been determined, in order configure an HF adaptive sidelobe canceller that meets military requirements, several key problems must be resolved. One such problem involves the desired signal discriminant. That is, a method that must be employed to prevent the array controller from nulling the desired signal. In the HF band, many sources of interference are indistinguishable from the desired signal on a time and/or frequency basis.
A second problem relates to the use of multiple loops in a dense signal/interference environment. Techniques must be used to bandlimit the operation of the array controller to the particular channel-frequency of interest. If bandlimiting is not employed, the degrees of freedom associated with the adaptive array may be used up on essentially harmless out-of-channel interferers. The bandlimiting approach selected must be capable of supplying the proper error signals to a set of multiple array weights so that the processing capability of the array can be totally and efficiently used.
A further problem relates to dynamic range performances. Both signals and interferers in the HF band can normally possess dynamic ranges in excess of 100 dB. A requirement for controller operation must be defined for performance over the expected power levels.
Finally, there is the problem of processed contrast ratio. It is assumed that no processing gain is available for the desired signal. The controller itself must operate to supply a processed S/J ratio which is adequate fo extracting the required information.
Although these problems can be addressed and resolved to a degree by state of the art radar technology, excessively complex hardware implementations are generally required. This is particularly true of conventional multiplexed adaptive antenna techniques that utilize signals from all elements simultaneously. This deficiency has been overcome by providing a system in which one reference receiver and multiplexed correlator perform an equivalent function of the multiple receiver used in the conventional system. This system is described in my co-pending patent application, Ser. No. 623,288 entitled Multiplexed Adaptive Array System, filed on even date herewith.
The prior art systems and the system described in my co-pending patent application operate in a wide dynamic range environment and require an automatic gain control (AGC) system that will maintain a stable, adaptive loop gain at all signal input levels. Conventional AGC systems have not been effective in accomplishing this. The present invention is directed toward providing an AGC circuit that will solve this problem and provide constant gain over the entire cancellation range.